Field of the Invention
The present invention relates to a method of manufacturing a rotary scale to be used for a rotary encoder.
Description of the Related Art
As means for detecting a movement amount and a rotation amount of a measurement object, encoders have been known. Examples of the encoders include optical encoders, magnetic encoders, and capacitive encoders. The optical encoders are constituted by a light source, a scale that reflects or transmits light emitted from the light source and is displaceable relatively to the light source, and a light-receiving element that receives the light reflected by or transmitted through the scale.
On the scale used for the optical encoders, a pattern that reflects or transmits the light is formed. Depending on the relative displacement of the scale, an amount of the light received by the light-receiving element varies. According to this characteristic, the optical encoders have a basic configuration in which they detect the displacement on the basis of a detection signal produced depending on the variation in the amount of the light received by the light-receiving element.
The optical encoders are broadly classified into rotary encoders and linear encoders, depending on a shape of the scale. The rotary encoders include an annular (doughnut-shaped) rotary scale assembled thereto whose coaxiality of the pattern is set to match those of a shaft, as a rotating shaft, and a hub. The rotary encoders detect, with an angle detection head, the displacement of the pattern formed on the rotary scale, thereby performing angle detection.
FIG. 10 is a diagram illustrating a problem concerning a rotation angle detection error caused by application of a conventional rotary scale to the rotary encoders. FIG. 10A is a schematic view of the conventional rotary scale whose outer shape has been processed. With reference to FIG. 10, a description will be given of a case of using plastic or glass as a substrate of a rotary scale 202. When the substrate of the rotary scale 202 is composed of plastic or glass, there is an advantage that a scale pattern 208 with a high resolution can be formed. However, when the substrate of the rotary scale is composed of plastic or glass, there are problems as illustrated in FIG. 10A such as a difficulty in achieving a high coaxiality and in highly accurately processing an outer shape 262 as a reference in the fixing of the rotary scale because the formation of the scale pattern 208 and the processing of the outer shape 262 are mutually different steps and both of plastic and glass are materials difficult to process.
In addition, when the rotary scale 202 having, as illustrated in FIG. 10A, a low coaxiality between the scale pattern 208 and the outer shape 262 and a low outer shape processing position accuracy is fixed to the shaft and the hub each as a rotating shaft 210 as illustrated in FIG. 10B, it is difficult to reduce an eccentricity amount ε2 between a center axis 241 of the shaft and the hub and a center axis 242 of the scale pattern 208. This difficulty has resulted in a problem that, as illustrated in FIG. 10C, it is difficult to reduce the rotation angle detection error caused by the rotary encoder with which the rotary scale is integrated.
There has been another problem that performing positioning adjustment of the scale pattern, and the shaft and the hub before fixing the rotary scale in which the scale pattern 208 and the outer shape 262 have a low coaxiality therebetween and which has a low outer shape processing position accuracy to the shaft and the hub requires a long period of time and a high cost for the positioning adjustment.
Japanese Patent Application No. (“JP”) 5132398 discloses a pulse code wheel 50 on which marks 210-c that allow a user to check an outer-shape processing accuracy of a marginal portion 203a of a fitting hole 203 into which the rotating shaft is to be fitted are formed as illustrated in FIG. 11. The pulse code wheel 50 disclosed in JP 5132398 allows the user to check the outer-shape processing accuracy by seeing the marks with use of a simple observation tool such as a loupe.
On the other hand, JP 2002-250640 discloses a high-accuracy rotary encoder that reads the rotary scale with multiple angle detection heads and calculates acquired data to reduce an influence of an eccentricity between the scale pattern and the outer shape.
However, employing an approach that, as in JP 5132398, checks the outer-shape processing accuracy with the observation tool such as the loupe, determines whether a processed outer shape is acceptable or not and then assembles the same leads to a low yield and an increase in cost because this approach requires a long period of time for the checking and has a low outer-shape processing accuracy.
On the other hand, reducing the influence of the eccentricity by the multiple angle detection heads as disclosed in JP 2002-250640 requires spaces to arrange the multiple angle detection heads. This requirement results in an increase in size of the rotary encoder and an increase in the number of component parts of the rotary encoder, leading to an increase in cost of manufacturing the rotary encoder.
Another possible method that integrally forms the scale pattern and the outer shape by using ultraviolet-curable plastic and thermosetting plastic also has had a difficulty in manufacturing the rotary scale having a high-resolution scale pattern.
Another example of a configuration that processes the scale pattern and an annular outer shape of the rotary scale while achieving a high coaxiality between them is a metal-etched scale, which has been conventionally commercially available. Since the scale pattern and the outer shape can be integrally processed, the metal-etched scale has an advantage that it can be manufactured such that an eccentricity amount between the scale pattern and the outer shape is small. However, forming a fine scale pattern on the metal-etched scale is difficult, which makes the metal-etched scale unsuitable for use as the high resolution rotary scale.